This invention relates generally to chart paper recording systems where data is recorded on moving chart paper as the paper is unwound from a rotatable supply roll and, more particularly, to an improved apparatus for preventing the paper from unwinding too fast. Chart recorders are, of course, well-known, and one common use of a chart recorder is in connection with densitometers of the type disclosed in U.S. Pat. No. 4,005,434, assigned to the Assignee of the present invention.
Densitometers are well-known devices which scan a sample and print a visible output signal on moving chart paper. The chart paper is initially tightly wound on a hollow core and is withdrawn from the core by a paper feed mechanism, which causes the paper and core to rotate.
One of the major problems associated with the feeding of paper is the need to unwind or withdraw the paper at a constant speed, and to stop the unwinding virtually instantaneously when the paper feed mechanism is stopped. In essence, the problem has two aspects. The first aspect is that the paper feed mechanism must provide sufficient force to overcome the inertia of a stationary paper supply roll to initiate rotation of the roll of paper. The second aspect of the problem is that the paper supply roll cannot rotate freely, i.e., drag must be provided to overcome the inertia of the rotating roll, so that when the paper feed mechanism stops, the rotation of the paper roll stops. If the paper roll rotates freely, or unwinds too fast, or does not stop when the paper feed mechanism stops, the paper can bunch up between the paper supply and the printing device. If the paper roll does not unwind fast enough, the paper feed mechanism can rip or tear the paper. The solution to these problems has been to maintain a constant external tension or drag on the paper supply to resist the force from the paper feed mechanism which tends to unwind the paper, and to thus stop the free rotation of the paper roll.
According to the prior art, an external spring loaded member was maintained in contact with the surface of the roll or supply of paper to create the drag on the paper. However, as the supply of paper was gradually used by unwinding the paper from the supply roll, the diameter of the remaining paper supply gradually decreased. Since the force from a spring is proportional to the working length of the spring, as the roll diameter decreased the working length of spring also decreased thus reducing the force or drag on the paper. Thus the prior art spring mechanism was unsatisfactory.
To overcome the problem of non-constant drag on the paper, the prior art provided a more complex tensioning device which consisted of a spring-loaded bar pressed against the roll of paper which bar has imparted to it an increasing amount of force as the chart paper roll diameter decreases to compensate for the decrease in the force from the spring, thus resulting in a net force on the paper which was substantially constant. The use of a complicated cam system was required in order to impart the increasing force on the tensioning device. This mechanism is inaccurate in that it created an uneven net pressure on the chart paper which was dependent upon the shape of the cam.
Thus the prior art, by attempting to modify spring-type tensioning devices by elaborate cam mechanisms, merely compounded the problem.
The tensioning device of the present invention overcomes the prior art difficulties of improper and imprecise tensioning of a cam and bar combination by totally eliminating all the external parts which provided the anti-rotational drag on the paper and by providing an entirely different approach to the problem of maintaining a constant drag on the chart paper. Specifically, the present invention maintains a constant drag on the roll of paper and does not require a direct external force on the surface of the paper.